By using scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), combined with thermodynamic theory calculation, the changes of the microstructure characteristics of solid SiO₂ cylinders immersed in CaCl₂ melt with incomplete dehydration at 1 173 K and their causes were studied. The effect of hydrolysis reaction of CaCl₂ salt on electrolysis characteristics of the solid SiO₂ was preliminarily analyzed. The results show that the CaCl₂ salt without strict dehydration operation is easy to hydrolyze at high temperatures. As long as the activity of the generated CaO in the melt is not less than 0.001, it can react with the SiO₂ to form a variety of silicates, such as monocalcium silicate (CS), tricalcium disilicate (C₃S₂) and dicalcium silicate (C₂S), step by step, which leads to a large change in the morphology and structure of the surface of the SiO₂ cylinder; the internal morphology changes little because of the low CaO content in the melt penetrated into the SiO₂ cylinder. The silicate layer on the surface may slow down the reaction rate and increase the difficulty of the reduction in the solid SiO₂ cylinder with only built-in cathode current collector.